Most photographic materials are manufactured such that the exposure balance between red, green and blue layers is attained when the intended light source is used. The intended light source in this case corresponds to the reference illuminant for which the photographic material's spectral response characteristics are determined. The two primary reference light sources are Daylight (5500 Kelvin) and Tungsten (3200 Kelvin). The tone scale and quality of the reproduction will depend upon the exposure level and color balance. In actual practice, the intended light source is seldom available and some variation is usually apparent. Further, the classic intended light sources have continuous spectra. Many of the light sources encountered today have significant line spectra included in the overall light spectra. Examples of this are fluorescent lamps and certain metal vapor lamps. It is difficult for the photographer to assess the nature of the light. A typical instrument used to determine the relative amount of red, green and blue light present is the Color Temperature Meter. This meter usually has three sensors, red, green and blue. The color temperature in Kelvin is recorded for the light source tested. The color temperature is then related through an empirical relationship to the spectral sensitivity of the photographic material. A difficulty with meters of this type is that they do not have any data for specific photographic materials, including the spectral sensitivity and the sensitometric curve (which determines the tonal relationship of the reproduction).
Manufacturers of filters for lenses and colored materials to filter light sources have tolerances around the performance of their products. A photographer usually has to accept the published parameters of the filter in the use of the product. The filters also can degrade over time or with storage conditions (adverse heat or light degradation). Thus, the exact nature of the filter product is not known.
The combination of light source variation and filtration variation can result in inconsistent exposure and consequent reproduction quality.
Photographic media have a wide variety of sensitometric response curves. No currently available light measuring system can easily relate the scene light source and areas of different light intensities with the sensitometric curve shape of the actual photographic material. Additionally, the effect of the spectral transmittance characteristic of the camera taking lens on color balance in the resultant exposure is also not addressed by the color temperature meters currently employed.
The problem to be addressed herein is the accurate electronic emulation of the photographic system such that the performance of the photographic material can be easily assessed with regard to light source, filters, lenses and product to product sensitometric differences.